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    <title>Document</title>
    <style>
        a:link {
            display: block;
            width: 93px;
            height: 29px;
            background-image: url('../images/btn/link.png')
        }
        
        a:hover {
            background-image: url('../images/btn/hover.png')
        }
        
        a:active {
            background-image: url('../images/btn/active.png')
        }
        /*
            图片属于网页中的外部资源，外部资源都需要浏览器单独发送请求加载，
                浏览器加载外部资源时是按需加载的，用则加载，不用则不加载
                像我们上边的练习link会首先加载，而hover和active会在指定状态触发时才会加载

                
          */
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